1. Technical Field
The present invention relates to a drive circuit, a physical quantity measuring device, and so on.
2. Related Art
In general, a physical quantity measuring device using a vibrator outputs a measurement signal proportional to the excitation current of the vibrator. In order for stable measurement, there is required a drive circuit or the like for keeping the amplitude of the excitation current constant irrespective of the temperature variation in the measurement environment. In JP-A-2008-261844 (Document 1) there is disclosed an invention of using an element identical to the resistor element of the detection circuit in the drive circuit to thereby cancel out the temperature variation between the excitation current and the detection current, thus stabilizing the output of the measurement signal.
However, as described in paragraphs 0060 through 0066 of Document 1, if the vibrational frequency of the quartz crystal vibrator is varied, the application thereof becomes difficult. According to Document 1, the problem can be solved by making the resistance value also identical. However, since the circuit design constraints are severe, it is not realistic. Further, the invention of Document 1 fails to go so far as considering the variation during the manufacturing process (hereinafter referred to as a manufacturing variation). For example, the capacitance of a capacitor, which is stable in the case of considering only the temperature characteristics, can naturally include the manufacturing variation. Therefore, in order for accurately detecting the minute variation in the measurement signal, it is required to design the drive circuit of the vibrator taking not only the temperature characteristics but also the manufacturing variation and the variation in frequency into consideration.